The present invention relates to packet data mobile communication systems; and, more particularly, to methods of scheduling transmission of packets on a time shared downlink packet data channel.
The demand for wireless data services, such as mobile Internet, video streaming, and voice over IP (VoIP), have led to the development of high speed packet data channels to provide high data rates needed for such services. High speed packet data channels are employed on the forward link in a variety of mobile communication systems, including IS-2000 (also known as 1xEV-DV), 1xEV-DO (also known as IS-856, Rev. A, or simply IS-856A), and Wideband Code Division Multiple Access (WCDMA) systems. The high speed packet data channel is a time shared channel, with downlink transmissions, e.g., from a base station to mobile stations, time-multiplexed and typically transmitted at full power.
Deciding which mobile station to serve at a given time is the function of a “scheduler.” A number of different scheduling strategies can be used, each with a different implication for system throughput and fairness. Typical scheduling strategies include round-robin scheduling, maximum throughput scheduling, and proportional fairness scheduling. In round-robin scheduling, the scheduler assigns the same number of time slots to all users, or assigns time slots on a first-come, first-serve basis, without taking into account channel conditions. Round-robin scheduling achieves a measure of fairness, but at the expense of lower throughput. Maximum throughput scheduling and proportional fairness scheduling, in contrast, attempt to increase system throughput as compared to round-robin scheduling by taking into account current channel conditions. A maximum throughput scheduler favors mobile stations with the best channel conditions and hence the highest supportable data rate to maximize system throughput. Proportional fairness scheduling tempers maximum throughput scheduling with a fairness criteria, so that mobile stations with bad channel conditions for an extended period can be served.
The scheduler makes scheduling decisions and selects the appropriate modulation and coding schemes based on channel feedback from the mobile stations. In 1xEV-DV and WCDMA systems, the mobile stations measure the quality of the forward packet data channel and transmit a channel quality indicator (CQI) to the base station. The base station maps the reported CQI value to one of a set of predefined modulation and coding schemes. The selected modulation and coding scheme determines the data rate for the downlink packet transmission used for scheduling (the “scheduling rate”). In IxEV-DO systems, the mobile stations measure the quality of the forward packet data channel and transmit a data rate control (DRC) value to the base station. The base station maps the DRC value to the “scheduling” rate for the downlink transmission. The scheduling rate is then used by the scheduler to make scheduling decisions.
Frequently, mobile stations are required to repeat each channel feedback report for a number of repetitions, in order to reduce the mobile station power required to make these reports. Thus, a given mobile station may be required to transmit each channel feedback report L times. Assuming each channel feedback report is transmitted in a separate slot, then the channel feedback reports have a minimum cycle time of L slots. However, the channel conditions may change more rapidly than every L slots. As such, the scheduling algorithms necessarily make scheduling decisions based on the most recent, but still possibly slightly out of date, channel feedback information in these situations.
It has been recognized that better link utilization of the downlink packet data channel may be achieved if more timely channel feedback information is used. Thus, one approach is to set L=1; however, this approach consumes greater power at the mobile station and loses the advantage of time diversity gains on the reverse link. Thus, L is typically set to a higher value, with the particular value established based on a tradeoff between the savings from higher L values and staleness of the channel feedback information. Further, the value of L is typically set on a sector wide basis, and based on the conditions of the user with the poorest reverse link conditions. Thus, it is possible that channel feedback reports from users with better reverse link conditions may be safely received after a smaller number of repetitions (i.e., <L). At present, the scheduler does not take advantage of this situation, but instead waits until all L repetitions are received before scheduling based on the new channel feedback report.